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Chapter 14 The Digestive System and Body Metabolism. The Digestive System and Body Metabolism. Digestion Breakdown of ingested food Absorption Passage of nutrients into the blood Metabolism Production of cellular energy (ATP). Organs of the Digestive System. Two main groups

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The Digestive System and Body Metabolism

  • Digestion
    • Breakdown of ingested food
  • Absorption
    • Passage of nutrients into the blood
  • Metabolism
    • Production of cellular energy (ATP)

Organs of the Digestive System

  • Two main groups
    • Alimentary canal
      • Continuous coiled muscular hollow tube
      • Performs digestive functions
        • Ingests, digests, absorbs, defecates
    • Accessory digestive organs
      • Teeth, tongue, glands
      • Assist in the digestive process

Organs of the Alimentary Canal

  • Mouth
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Anus

Mouth (Oral Cavity) Anatomy

  • Lips (labia) – protects the anterior opening
  • Cheeks – form the lateral walls
  • Hard palate – forms the anterior roof
  • Soft palate – forms the posterior roof
  • Uvula – fleshy projection of the soft palate

Mouth (Oral Cavity) Anatomy

  • Vestibule – space between lips and cheeks externally and teeth and gums internally
  • Oral cavity – area contained by the teeth
  • Tongue – attached at hyoid and styloid processes of the skull, and by the lingual frenulum

Mouth (Oral Cavity) Anatomy

  • Tonsils
    • Palatine tonsils
    • Lingual tonsil

Processes of the Mouth

  • Mastication (chewing) of food
  • Mixing masticated food with saliva
  • Initiation of swallowing by the tongue
  • Allowing for the sense of taste

Pharynx Function

  • Serves as a passageway for air and food
  • Food is propelled to the esophagus by two muscular layers
    • Longitudinal inner layer
    • Circular outer layer
  • Food movement is by alternating contractions of the muscle layers (peristalsis)


  • Runs from pharynx to stomach, through the diaphragm
  • Conducts food by peristalsis
    • Slow rhythmic squeezing
  • Passageway for food only
    • Respiratory system branches off after the pharynx

Layers of Alimentary Canal Organs

  • Mucosa
    • Innermost layer
    • Moist membrane that lines the lumen
      • Surface epithelium
      • Small amount of connective tissue (lamina propria)
      • Small smooth muscle layer

Layers of Alimentary Canal Organs

  • Submucosa
    • Just beneath the mucosa
    • Soft connective tissue with blood vessels, nerve endings, and lymphatics

Layers of Alimentary Canal Organs

  • Muscularis externa – smooth muscle
    • Inner circular layer
    • Outer longitudinal layer
  • Serosa
    • Outermost layer – visceral peritoneum
    • Layer of serous fluid-producing cells

Stomach Anatomy

  • Located on the left side of the abdominal cavity
  • Food enters at the cardioesophageal sphincter

Stomach Anatomy

  • Regions of the stomach
    • Cardiac region – near the heart
    • Fundus
    • Body
    • Phylorus – funnel-shaped terminal end
  • Food empties into the small intestine at the pyloric sphincter

Stomach Anatomy

  • Rugae – internal folds of the mucosa
  • External regions
    • Lesser curvature
    • Greater curvature

Stomach Anatomy

Figure 14.4a


Stomach Functions

  • Acts as a storage tank for food
  • Site of food breakdown
    • Physically- muscular contractions (peristalsis)
    • Chemical breakdown of proteins
  • Chyme (processed food) is then delivered to the small intestine
stomach contents
Stomach Contents
  • Gastric juice
    • Sight, smell & taste of food stimulates the parasympathetic nervous system to secrete gastric juice
    • Composition
      • Gastrin (Hormone)
      • Enzymes
      • Mucus
      • Hydrochloric acid

Specialized Mucosa of the Stomach

  • Simple columnar epithelium
    • Mucous neck cells – produce a sticky alkaline mucus
    • Gastric glands – secrete gastric juice
    • Chief cells – produce protein-digesting enzymes (pepsinogens)
    • Parietal cells – produce hydrochloric acid
    • Endocrine cells – produce gastrin

Structure of the Stomach Mucosa

  • Gastric pits formed by folded mucosa
  • Glands and specialized cells are in the gastric gland region

Small Intestine

  • The body’s major digestive organ
  • Site of nutrient absorption into the blood
  • Muscular tube extending from the pyloric sphincter to the ileocecal valve
  • Suspended from the posterior abdominal wall by the mesentery

Subdivisions of the Small Intestine

“Dogs Just Itch!”

  • Duodenum (5%)
    • Attached to the stomach
    • Curves around the head of the pancreas
  • Jejunum (~40%)
    • Attaches anteriorly to the duodenum
  • Ileum (~60%)
    • Extends from the jejunum to large intestine

Chemical Digestion in the Small Intestine

  • Duodenum
    • Source of enzymes that are mixed with chyme
      • Intestinal cells
      • Pancreas
  • Bile enters from the gallbladder

Villi of the Small Intestine

  • Fingerlike structures formed by the mucosa
  • Give the small intestine more surface area

Structures Involved in Absorption of Nutrients

  • Absorptive cells
  • Blood capillaries
  • Lacteals (specialized lymphatic capillaries)

Microvilli of the Small Intestine

  • Tiny projections of the plasma membrane
  • Found on absorptive cells
  • “Brush border”

Circular Folds of the Small Intestine

  • Circular folds or plicae circulares
    • Deep folds of the mucosa and submucosa
    • Do not disappear when food fills the small intestine

Digestion in the Small Intestine

  • Enzymes from the brush border
    • Break complex sugars into simple sugars
    • Complete some protein digestion
  • Pancreatic enzymes play the major digestive function
    • Help complete digestion of starch (pancreatic amylase)
    • Carry out about half of all protein digestion (trypsin, etc.)

Digestion in the Small Intestine

  • Pancreatic enzymes play the major digestive function (Continued…)
    • Responsible for fat digestion (lipase)
    • Digest nucleic acids (nucleases)
    • Alkaline content neutralizes acidic chyme

Absorption in the Small Intestine

  • Water is absorbed along the length of the small intestine
  • End products of digestion
    • Most substances are absorbed by active transport through cell membranes
    • Lipids are absorbed by diffusion
  • Substances are transported to the liver by the hepatic portal vein or lymph

Propulsion in the Small Intestine

  • Peristalsis is the major means of moving food
  • Segmental movements
    • Mix chyme with digestive juices
    • Aid in propelling food

Large Intestine

  • Larger in diameter, but shorter than the small intestine
  • Major Functions
    • Absorption of water
    • Elimination of indigestible food from the body as feces
    • Does not participate in digestion of food
    • Goblet cells produce mucus to act as a lubricant

Large Intestine

Figure 14.8


Structures of the Large Intestine

  • Cecum
    • Saclike
    • First part of the large intestine
  • Appendix
    • Accumulation of lymphatic tissue that sometimes becomes inflamed (appendicitis)
    • Hangs from the cecum

Structures of the Large Intestine

  • Colon
    • Ascending
    • Transverse
    • Descending
    • S-shaped sigmoidal
  • Rectum
  • Anus
    • External body opening

Food Breakdown and Absorption in the Large Intestine

  • No digestive enzymes are produced
  • Resident bacteria digest remaining nutrients
    • Produce some vitamin K and B
    • Release gases
  • Water and vitamins K and B are absorbed
  • Remaining materials are eliminated via feces

Propulsion in the Large Intestine

  • Sluggish peristalsis
  • Mass movements
    • Slow, powerful movements
    • Occur three to four times per day
  • Presence of feces in the rectum causes a defecation reflex
    • Internal anal sphincter is relaxed
    • Defecation occurs with relaxation of the voluntary (external) anal sphincter

Accessory Digestive Organs

  • Salivary glands
  • Teeth
  • Pancreas
  • Liver
  • Gall bladder

Salivary Glands

  • Saliva-producing glands
    • Parotid glands
      • Located anterior to ears
    • Submandibular glands
    • Sublingual glands
  • Saliva
    • Mixture of mucus and serous fluids
    • Helps to form a food bolus
    • Contains salivary amylase to begin starch digestion
    • Dissolves chemicals so they can be tasted
    • Contains lysozyme & antibodies (IgA)  inhibit bacteria


  • Role
    • Masticate (chew) food
  • Humans have two sets of teeth
    • Deciduous (baby or milk) teeth
      • 20 teeth- Fully formed by age two
    • Permanent teeth
      • Replace deciduous teeth beginning between the ages of 6 to 12
      • A full set is 32 teeth, but some people do not have wisdom teeth


  • Produces a wide spectrum of digestive enzymes that break down all categories of food
  • Enzymes are secreted into the duodenum
  • Alkaline fluid introduced with enzymes neutralizes acidic chyme coming in from the stomach
  • Endocrine products of pancreas
    • Insulin
    • Glucagons


  • Largest gland in the body
  • Located on the right side of the body under the diaphragm
  • Consists of four lobes suspended from the diaphragm and abdominal wall by the falciform ligament
  • Connected to the gallbladder via the common hepatic duct


  • Produced by cells in the liver
  • Composition
    • Bile salts  Emulsify fats
    • Bile pigment (mostly bilirubin from the breakdown of hemoglobin)
    • Cholesterol
    • Phospholipids
    • Electrolytes
    • No enzymes

Role of the Liver in Metabolism

  • Several roles in digestion
  • Detoxifies drugs and alcohol
  • Degrades hormones
  • Produce cholesterol, blood proteins (albumin and clotting proteins)
  • Plays a central role in metabolism


  • Sac found in hollow fossa of liver
  • Stores bile from the liver by way of the cystic duct
  • Bile is introduced into the duodenum in the presence of fatty food
  • Gallstones can cause blockages

Processes of the Digestive System

  • Ingestion
    • Getting food into the mouth
  • Propulsion
    • Moving foods from one region of the digestive system to another
    • Peristalsis – Involuntary, alternating waves of contraction
    • Segmentation – moving materials back and forth to aid in mixing

Processes of the Digestive System

  • Propulsion continued…
    • Peristalsis
    • Segmentation

Processes of the Digestive System

  • Mechanical digestion
    • Mixing of food in the mouth by the tongue
    • Churning of food in the stomach
    • Segmentation in the small intestine

Processes of the Digestive System

  • Chemical Digestion
    • Enzymes break down food molecules into their building blocks
    • Each major food group uses different enzymes
      • Carbohydrates are broken to simple sugars
      • Proteins are broken to amino acids
      • Fats are broken to fatty acids and alcohols

Processes of the Digestive System

  • Absorption
    • End products of digestion are absorbed in the blood or lymph
    • Food must enter mucosal cells and then into blood or lymph capillaries
  • Defecation
    • Elimination of indigestible substances as feces

Control of Digestive Activity

  • Mostly controlled by reflexes via the parasympathetic division
  • Chemical and mechanical receptors are located in organ walls that trigger reflexes

Control of Digestive Activity

  • Stimuli include:
    • Stretch of the organ
    • pH of the contents
    • Presence of breakdown products
  • Reflexes include:
    • Activation or inhibition of glandular secretions
    • Smooth muscle activity

Nutrition - Take a Class!

  • Nutrient – substance used by the body for growth, maintenance, and repair
  • Categories of nutrients
    • Carbohydrates: simple sugars, starches, fiber
    • Lipids: triglycerides, phospholipids, fatty acids
    • Proteins: amino acids
    • Vitamins
    • Mineral
    • Water

Body Energy Balance

  • Energy intake = total energy output (heat + work + energy storage)
    • Energy intake is liberated during food oxidation
    • Energy output
      • Heat is usually about 60%
      • Storage energy is in the form of fat or glycogen